Don't Be Uptight

Most electrical equipment requires physical connection to electrical conductors as a major part of the installation process. These single- and multiple-conductor connections can be mechanical-pressure connectors, such as a mechanical set screw device, wire binding screws for smaller sizes of conductors, as well as mechanical terminal lugs, compression lugs with single or double holes and short or long barrels, and split-bolt connectors with either single or double bolts. For same-size multitap or splice-reducing connections, power-distribution blocks and insulated-mechanical connectors can be used.

With all these varied connectors, one of the most important issues is ensuring the connection is reliable for both the short time period immediately following the installation and over the longer operating life of the equipment. Failure to ensure the proper conductor connection, due to undertorquing or overtorquing of mechanical connections, can result in improper operation of the equipment, costly outages, possible fires and a host of other negative results.

Electrical equipment often has listing requirements for the proper tightening or torquing of connections. The marking for the torque levels for connection points may be located either on or in the equipment. The torquing requirements may also be located in the written listing and operational requirements for the equipment, not marked on the actual equipment. Section 110.3(B) in the National Electrical Code (NEC) states that listed or labeled equipment must be installed and used in accordance with any instructions included in the listing or labeling, which would include any torquing requirements that are a part of the listing and labeling. Furthermore, 110.14(A) states, “connection of conductors to terminal parts shall ensure a thoroughly good connection without damaging the conductors and shall be made by means of pressure connectors (including set-screw type), solder lugs, or splices to flexible leads. Connection by means of wire-binding screws or studs and nuts that have upturned lugs or the equivalent shall be permitted for 10 AWG or smaller conductors.” Though specific torquing requirements are not provided in 110.3(B) and 110.14(A), all electricians know proper torquing is a critical part of any equipment connection.

In the 2008 and previous editions of the NEC, the only location of specific torque requirements was 430.9(C), which stated: “Control circuit devices with screw-type pressure terminals used with 14 AWG or smaller copper conductors shall be torqued to a minimum of 0.8 N·m (7 lb.-in.) unless identified for a different torque value.” In the torque value, the “N” stands for Newton, which is the international system’s unit for force. In other words, it is equal to the amount of net force required to accelerate a mass of one kilogram at a rate of 1 meter per second squared (kilogram meter per second squared). Compliance with this requirement in 430.9(C) can only be reliably and accurately measured with a torque screwdriver that is either calibrated in metric or in inch-pound values.

However, with the adoption of the 2011 NEC, a new Informative Annex I has been inserted to cover the recommended tightening torques for conductor terminations. These three tables in Annex I are to be used for power and lighting circuits where connector or equipment manufacturer’s recommended torque values are not available. Control and signal circuits may require different torque values, so for these applications, contact the connector manufacturer.

Table I.1 covers the tightening of torques for screws that are slotted head No. 10 and larger, as well as any hexagonal-head screws tightened by an external socket drive wrench. For values of slot widths or lengths that do not correspond to the specific values provided in Table I.1, select the largest torque value associated with the specific conductor size. The slot width is the nominal design value for that particular screw.

Table I.2 covers the tightening of torques for slotted-head screws smaller than No. 10 intended for use with 8 AWG (8.4 square mm) or smaller conductors. The slots in the screw heads are measured from the top of the slot to the bottom. With slot lengths of intermediate values (lengths that are not specific to the actual values shown in Table I.2), the torque value would be based on the next shorter length of slot that is shown in the table. In other words, the slot length would be rounded down to the closest lower length, and that torque value would be used.

Table I.3 covers the tightening torque for screws with recessed Allen or square drives. Each table has a Column A for torquing terminals for current-cycling during testing and a Column B for field-installation applications for the various types of wire connectors. Do not use the Column A torquing values for field applications.

ODE is a staff engineering associate at Underwriters Laboratories Inc., based in Peoria, Ariz. He can be reached at 919.949.2576 and mark.c.ode@us.ul.com.